Most decorative light strings are series-parallel light strings having multiple groups of series-connected lights connected together in parallel. In a series-parallel string, the voltage at each light is the source voltage divided by the number of lights in the series group. For example, one commonly-used decorative light string includes two groups of 50 lights connected in series to form a 100-count light string. When connected to a 120 VAC source, the voltage at each bulb of a 50-bulb series group is approximately 2.4 VAC. Because of the series construction, if any one light in the series group fails, all lights in the series group lose power.
Typically, such light strings include a power plug at one end and a power receptacle, also referred to as an end connector, at the opposite end, for connecting light strings end-to-end. The power plug typically includes a pair of wires, a lead wire and a return wire, contacting a pair of terminals for plugging into a power source. The power plug may also include an additional power receptacle on the back of the power plug so that multiple plugs may be powered at the same power outlet by plugging one plug into another.
The lead wire of the power plug connects to the first light in the series group. Multiple short sections of wire connect individual lights in series. Each end of the short wire is stripped of insulation, crimped to a conducting terminal, and inserted into a lamp holder. The long return wire extends the length of the series group, intertwined with the shorter wires, and connects at the last light. Most lamp holders of the series group receive two wires to wire the individual light in series, while the first and last lamp holders of each series receive three wires. A second series group may be added to the first, and an additional wiring connections may be made to add 10 the power receptacle at the end of the series.
Most pre-lit artificial trees include multiple light strings of this common series-parallel connected end-to-end, or by stacking plugs. Modern pre-lit artificial trees may include as many as 1,000 or 1,500 lights, or ten to fifteen 100-light strings, with the actual number varying depending on tree size, desired lighting density, and so on. With the large number of lights and light strings, it can be difficult to find and then properly connect the necessary plugs in order to power all of the light strings on the tree. Light strings may be connected to one another within a given tree section, or sometimes between sections, by connecting the strings end to end or by stacking plugging. Short extension cords may be strung along the outside of the trunk to carry power to the various interconnected light strings. The result is a complex web of lighting that often requires a consumer to not only interconnect the plugs and receptacles of individual light strings together, but to stack and plug multiple light strings and cords into multiple power outlets.